This application relates to improvements in a ram air scoop for use on a gas turbine nacelle.
Gas turbine engines are known, and typically include a fan delivering a portion of air into a core engine leading to a compressor. The compressor compresses the air and delivers it into a combustor where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, driving the turbine rotors to rotate.
Another portion of the fan's air is delivered into a nacelle, or outer housing which defines a bypass air flowpath between an outer core engine housing and the outer housing of the nacelle. This bypass air provides propulsion for an aircraft that mounts the gas turbine engine.
Historically a low pressure turbine has driven a low pressure compressor and the fan generally at the same speed. More recently, it has been proposed to incorporate a gear drive between the low pressure compressor and the fan such that the two can rotate at different speeds. With this advancement, the bypass duct has become significantly larger.
A portion of the bypass air is tapped for use as cooling air at various locations in the engine. Flush inlets and holes have been provided generally in the inner wall of the nacelle, or the outer core engine housing, to provide this cooling air. However, with the larger bypass ducts, and the change in fan speed, ram air scoops may be required. There are challenges with such scoops, particularly at the inlet, due to boundary layer issues in the nacelle.
In particular, FIG. 2 shows a nacelle 15 having a nacelle outer wall 80 spaced from an inner wall 82. Inner wall 82 may be a core engine outer wall. In the prior art, there have been cooling air taps 84 spaced at various locations in the nacelle 15.
Scoop air inlets such as 86 have been incorporated into the inner wall 82 of the nacelle to provide cooling air to various systems and heat exchangers on the gas turbine engine. An inlet 88 taps a portion of the bypass air B.
FIG. 3 shows a concern with such a prior art scoop 86. A boundary layer 90 is created as the bypass air approaches the inlet 88. As the bypass air enters the inlet 88, there is flow reversal 93 at areas immediately adjacent to an outer surface of the inner wall 82, such as surface 99 of a portion of the scoop 86 leading into the inlet 88. Flow reversal 93 causes a region of flow separation 94 downstream of the inlet 88, and limits the amount of air passing at 96 to a downstream user 98 of the cooling air.